This invention relates to a process and an apparatus for the manufacture of fibers of mineral materials such as glass, delivered through orifices in the bottom of a bushing. It is especially useful in the manufacture of long discontinuous fibers used in the formation of a sheet or pad or gathered into the form of a band or cord; and the invention provides for increase in the number of orifices in the bushing and thus of the number of attenuable streams delivered from the bushing.
The most commonly used technique of fiberization for the manufacture of this type of fibers consists of subjecting a multiplicity of streams of molten vitreous material to the action of an attenuating gas stream. These glass streams issue from a multiplicity of small tubes, each having a single orifice and arranged so that the streams project from the lower surface of the bottom of a bushing.
These tubes, commonly known as tips, provide for the stable separation of the streams of material and generally prevent the well-known phenomenon of "flooding", i.e., spreading of the molten material over the lower surface of the bottom of the bushing.
Such a process of manufacture has been described in particular in the following U.S. Pat. Nos: 2,206,058; 3,207,587; 3,607,164 and 3,836,346.
Prior techniques for carrying out this process of manufacture all have certain features in common, as follows:
The bushing used, generally made of a platinum alloy, has a flat bottom in the form of a very elongated rectangle provided with one to three longitudinal rows of tips. The bushing provides for maintenance of the desired temperature of the molten material being fiberized and may also be relied upon for melting the material, in which latter event, the vitreous material is introduced into the bushing in solid state, these techniques being already well-known to the man skilled in the art.
Two blowers have commonly been used, arranged one on each side of the long edges of the bottom of the bushing and each directing a gas at or below the level of the said bottom. The patents cited above give examples of such blowers placed below the bottom of the bushing; these blowers have slots or nozzles directed downwardly and converging on the vertical plane passing through the longitudinal axis of the said bushing.
The streams of glass issuing from the tips are attenuated into fibers, usually long discontinuous fibers, under the action of the gas currents leaving these slots or nozzles at a high velocity.
The prior art arrangements have two major disadvantages which limit the life of each bushing and the quantity of fibers produced from it.
The tips which channel the glass at the outlet of the bushing occupy a considerable amount of space at the bottom of said bushing; but in order that the action of the gas currents may be effective, the width, and consequently, the useful surface area of the bottom of the bushing is necessarily minimized. Due to these two limitations, the number of tips per bushing is not much more than about 100 in the arrangements of the prior art.
Furthermore, the gas currents used for attenuation cause a strong induction of air along and between the tips. The consequent air friction over the walls of these tips results in rapid wear and considerably shortens the life of the bushing.
U.S. Pat. No. 3,836,346 proposes a solution for overcoming one of the above-mentioned disadvantages. It consists of using tips which are perforated asymmetrically in such a manner that the portion of the lateral wall presented outwardly is thicker than that presented towards the vertical plane passing through the longitudinal axis of the bushing. While not eliminating the erosive action of the violent air currents produced during fiberization, this structure delays the consequences and increases the life of the bushings. This solution does not provide for an increase in the density of the orifices.
The present invention has as its object a process and an apparatus capable of improving the manufacture of fibers, particularly relatively long but discontinuous fibers.
It has more particularly the object of increasing the number of streams of molten material capable of being attenuated from a bushing and, at the same time, increasing the life of the bushing.
These objects are achieved by the use of a novel form of bushing in a fiberizing installation such as that defined below.
The fiberizing installation essentially comprises a source of thermoplastic material such as glass, a bushing of elongated form heated by Joule's effect, apparatus for attenuation by blowing arranged to cause gas flow over the bottom surface of the bushing and also downwardly from the bottom surface to effect attenuation of glass streams leaving the perforations in the bushing. In certain embodiments, blowers are placed parallel to the vertical plane passing through the longitudinal axis of the bushing and on both sides of the bottom of said bushing, with the openings of said blowers directed downwardly. Means for receiving the fibers obtained are also provided.
The bottom of the bushing according to the invention has a substantially continuous wall area or areas perforated by a multiplicity of orifices, without the presence of tips.
According to one of the characteristics of the invention, the bottom of the bushing is equipped with a wall having an upper surface hollowed out by cells or alveoles, each alveole having a lower wall perforated by a plurality of orifices, while the lower surface of the wall preferably has grooves cut into it to provide a separation of the lower wall surface surrounding the orifices for one alveole from the lower wall surface surrounding the orifices for adjacent alveoles.